PROJECT SUMMARY It is unknown how myeloid or endothelial cell expression of CXCR2 contributes to myeloid-derived suppressor cell (MDSC) recruitment to the lungs, with subsequent pulmonary vascular remodeling. There is an urgent need to close this gap in knowledge because, until accomplished, immunotherapeutic modulation of CXCR2 contribu- tion to the development of vascular remodeling, and pulmonary hypertension (PH), will likely remain beyond reach. The overall objective here is to define the contribution of MDSC recruitment through chemokine receptor CXCR2, expressed by either circulating myeloid cells or the pulmonary vascular endothelium. The central hy- pothesis is that tissue specific CXCR2 expression is necessary for polymorphonuclear (PMN)-MDSC recruitment to the pulmonary vasculature and PH development. The scientific premise for this hypothesis has been formu- lated on the basis of preliminary data demonstrating that MDSCs expressing CXCR2 are necessary for devel- opment of PH in animal models of pulmonary vascular disease, and that this circulating cell population is present to a higher degree in whole blood of idiopathic pulmonary fibrosis (IPF) patients with PH, compared to IPF pa- tients without elevated pulmonary pressures. The rationale for the proposed research is that, upon completion of experiments, future studies can be proposed taking advantage of University of Florida expertise in tissue- specific delivery of either existing CXCR2 inhibitors though use of nanoparticle technology, or CXCR2 directed gene-therapy utilizing AAV vectors. Expected outcomes, as a consequence of proposed work, are a vertical advancement in the understanding of CXCR2 contribution to vascular remodeling, additionally establishing a foundation for future studies detailing mechanisms of MDSC mediated PH development related to chronic lung disease, such as IPF. The results are expected to have positive translational impact because it is probable that the identified tissue-specific CXCR2 influence will provide targets for immunotherapeutic interventions. The cen- tral hypothesis will be tested by pursuing the following specific aims: 1) test the hypothesis that CXCR2 expres- sion by PMN-MDSC promotes development of PH, and 2) test the hypothesis that vascular endothelial cell CXCR2 expression is protective against MDSC recruitment in PH. In the first aim, wild type and transgenic mice with tissue-specific deletion of CXCR2 in myeloid-derived cells (LysM.Cre-CXCR2fl/fl, mCXCR2 mice) will be used to determine the effect that attenuated MDSC trafficking will have on the development of PH in two models of disease (bleomycin-induced pulmonary fibrosis and chronic hypoxia). In the second aim, transgenic mice with tissue-specific deletion of CXCR2 in vascular endothelial cells (VECad.Cre-CXCR2fl/fl, eCXCR2 mice) will be used in the bleomycin and hypoxia models of PH. The contribution of these studies will be significant because it represents a strategy to improve clinical outcomes through prevention of CXCR2-mediated trafficking to the lung. The proposed research is innovative because it represents a departure from the status quo by shifting focus from vasodilator therapy to an immunoregulatory mediator, MDSCs, in the pathobiology of PH.